PROJECT SUMMARY/ABSTRACT Nearly 1 in 3 children and adolescents ? 23 million youth ? in the United States are affected by overweight or obesity, and low-income, racial/ethnic minority and rural children are disproportionately impacted. Beyond the adverse impact of obesity on physical, social, and emotional wellbeing in childhood, children with obesity are at greater risk of obesity in adulthood, placing them at higher risk of obesity-related cancers, cardiometabolic disease, and early mortality. Despite guidance from the US Preventive Services Task Force and expert committees, pediatric primary care clinicians often do not follow available guidance for managing children with obesity. Electronic health records (EHRs) have the potential to improve clinicians' diagnosis and management of obesity by providing tools such as reminders and clinical decision support. We have demonstrated that an intervention package, consisting of EHR-based decision support for pediatricians and educational materials for parents, improved BMI among children in two clinical trials at a multisite group practice in Massachusetts. We have previously estimated, using a simulation model of the U.S. population, that high-fidelity dissemination and implementation of this intervention package among pediatric primary care practices with fully functional EHRs over 10 years has the potential to reach 2 million children with obesity and avert 43,000 cases of obesity at 10 years. However, little is known regarding the effectiveness of the intervention in various pediatric primary care settings, particularly those serving children from populations disproportionately impacted by obesity, or regarding the specific factors that influence the successful adoption and implementation of these tools. In partnership with a national practice-based research network, we aim to capitalize on advances in the field of Implementation Science to apply and evaluate this intervention for childhood obesity among a sample of 84 primary care practices affiliated with three health systems serving children with racial/ethnic, socio-economic, rural-urban, and geographic diversity. The scientific objectives of the proposed award are (1) to optimize a stakeholder-informed intervention and implementation package tailored to site-specific context among a national sample of participating practices, (2) to evaluate the impact of the intervention and implementation facilitation package in improving obesity-related care delivery and children?s BMI, and (3) to rigorously examine what works where and why among participating practice sites. To achieve these aims, we have assembled a transdisciplinary team with expertise is obesity, implementation science, clinical informatics, mixed methods, multi-site interventions, and user-centered design. We will apply a Hybrid Type 2 effectiveness-implementation study design wherein we will employ mixed methods in our formative and summative evaluation plan. To facilitate broad-scale dissemination in future implementation efforts, the results from this proposed study will provide invaluable evidence on the effectiveness of the intervention package and implementation facilitation strategy in settings serving populations most heavily burdened by obesity.